1. Field of the Invention
The present invention relates to an organic light emitting device, and more particularly, to an organic light emitting device in which a photonic crystal concavo-convex structure is formed in a transparent substrate and an transparent electrode layer to increase a light extraction efficiency.
2. Description of the Related Art
Recently, display devices based on organic light emitting materials become the center of attraction due to flatness, high definition, portability, low power consumption and so forth.
FIG. 1 is a schematic view of a conventional organic light emitting device. Referring to FIG. 1, the conventional organic light emitting device has a structure in which a transparent electrode layer 20, a hole conduction layer 30, an electron conduction layer 40 and a cathode layer 50 are sequentially stacked on a transparent substrate 10.
Herein, a glass substrate is typically used as the transparent substrate 10. An ITO (Indium-Tin-Oxide) layer is mainly used as the transparent electrode layer 20. In addition, an Mgxe2x80x94Al alloy layer may be used as the cathode electrode layer 50. The hole conduction layer 30 and the electron conduction layer 40 are comprised of an organic EL(electroluminescent) material. Typically, the hole conduction layer 30 is comprised of N,Nxe2x80x2-diphenyl-N,Nxe2x80x2-bis-(3-methylphenyl)-4,4xe2x80x2-diamine (hereinafter, xe2x80x9cTPDxe2x80x9d) or polyethylenedeoxythiophene (PEDOT), and the electron conduction layer 30 is widely comprised of tris (8-hydroxyquinolino) aluminum (hereinafter, xe2x80x9cAlq3xe2x80x9d). The typical material of each layer has an absolute refractive index, i.e., n(glass)=1.46, n(ITO)=1.8, n(TPD)=1.76 and n(Alq3)=1.7.
As shown in FIG. 1, when a negative voltage is applied to the cathode electrode layer 50 and a positive voltage is applied to the transparent electrode layer 20, the combination of a hole and an electron is occurred in a junction portion (35: hereinafter, xe2x80x9cactive areaxe2x80x9d) of the hole conduction layer 30 and the electron conduction layer 40. Thus, light is spontaneously radiated.
The light generated at the active area 35 is radiated through in turn an interface of the hole conduction layer 30 and the transparent electrode layer 20 and an interface of the transparent electrode layer 20 and the transparent substrate 10 to air. Since the absolute refractive index (n(ITO)=1.8) of the transparent electrode layer 20 is larger than that (n(Alq3)=1.7) of the electron conduction layer 40, at the interface of the electron conduction layer 40 and the transparent electrode layer 20, most of the light is refracted toward the transparent electrode layer 20 and then transmitted through the transparent electrode layer 20.
However, since the absolute refractive index (n(glass)=1.8) of the transparent electrode layer 20 is larger than the refractive index of substrate layer 10 (n(glass)=1.46), the light, at an angle larger than a critical angle, is totally reflected so as to be not transmitted to the glass. Further, since the absolute refractive index of the transparent substrate 10 is 1.46 and the absolute refractive index of the air is 1, the same phenomenon occurrs at the interface of the transparent substrate 10 and the air.
In the drawing, a reference symbol xcex8cc designates a critical angle between the transparent electrode layer 20 and the transparent substrate 10, and a reference symbol xcex8c is a critical angle between the transparent substrate 10 and the air, and xcex8o is an incident angle of the light which is incident to the transparent substrate 10 to be converted into the angle of xcex8c.
Assuming that the distribution of light generated from a specific radiation point of the active area 35 is spacially isotropic and the light is not reabsorbed, the amount of light, that is totally reflected from the transparent substrate, can be calculated by a following equation:       ∫          θ      ⁢              xe2x80x83            ⁢      o              θ      ⁢              xe2x80x83            ⁢      cc        ⁢                    T        glass            ⁡              (        θ        )              ⁢    sin    ⁢          xe2x80x83        ⁢    θ    ⁢          xe2x80x83        ⁢                  ⅆ        θ            .      
It is about 31.5%, wherein Tglss(xcex8) is a transmittance of the transparent substrate 10. And, in the same condition, the amount of light, that is totally reflected from the transparent electrode layer 20, can be calculated by a following equation:       ∫          θ      ⁢              xe2x80x83            ⁢      cc        90    ⁢                    T        ITO            ⁡              (        θ        )              ⁢    sin    ⁢          xe2x80x83        ⁢    θ    ⁢          xe2x80x83        ⁢                  ⅆ        θ            .      
It is about 51%, wherein TITO(xcex8) is a transmittance of the transparent electrode layer 20. To summarize, the total-reflected light amount is about 80%.
Therefore, in conventional organic light emitting devices, the light extraction efficiency is only about 20%. There is a big room for improvement. Because of the low light extraction efficiency, the power dissipation should be large, and the life time of the arrayed light emitting device is reduced. Therefore, one need to make the light extraction efficiency as large as possible.
In order to increase the light extraction efficiency, several schemes have been proposed. For example, a cone-shaped array is formed on a glass substrate, such that the light, entering at larger angles than the critical angle, can be transmitted to an outside (cf. High external quantum efficiency organic light emitting device, G. Gu, D. Z. Garbuzov, P. E. Burrows, S. Venkatesh, S. R. Forrest, Optics Letters, 22, 396, 1997). Or a laminated lens array is formed on a glass substrate to reduce the incident angle, thereby increasing the light extraction efficiency (cf. Improvement of output coupling efficiency of organic light emitting diodes by backside substrate modification, C. F. Madigan, M. H. Lu, J. C. Sturm, Applied Physics Letters, 27, 1650, 2000). In these methods, however, there are some problems related to fabricating methods and the image quality is poor.
Therefore, the object of the present invention is to provide an organic light emitting device in which a photonic crystal concavo-convex structure is formed in the transparent substrate 10 and the transparent electrode layer 20, thereby increasing the light extraction efficiency.
To achieve the aforementioned object of the present invention, the photonic crystal organic light emitting device includes a transparent substrate having a concavo-convex structure in an upper surface thereof, a transparent electrode layer formed on the transparent substrate, a hole conduction layer comprised of an organic EL material and formed on the transparent electrode layer, an electron conduction layer comprised of an organic EL material and formed on the hole conduction layer, and a cathode layer formed on the electron conduction layer.
Preferably, the lattice constant of the concavo-convex structure formed in the upper surface of the transparent substrate ranges from ⅓xcex to 2xcex, where xcex is the wavelength of light in the active area. And transparent electrode layer has a thickness of 30-200 nm. Further, the depth of a concave potion is formed as deep as possible within the extent that the electrical properties of the transparent electrode layer are acceptable.
The photonic crystal of the concavo-convex structure formed in the upper surface of the transparent substrate can be periodically and repeatedly arrayed in a square lattice type, a triangular lattice type or a honeycomb lattice type. The concavo-convex structure can be formed by etching the upper surface of the transparent substrate. However, it is clear that other methods such as a wet etching or a micro-imprinting also can be used.
If one wants to obtain the constant diffraction angle irrespective of the color of light generated at the interface between the electron conduction layer and the hole conduction layer, it is better to have a constant value xcex/xcex94, wherein xcex94 is a period of the photonic period due to the concavo-convex structure formed in the transparent substrate, and xcex is a wavelength of light incident on the photonic crystal.